This invention relates to laminating methods and particularly to a method for use in one-sided or single-sided lamination and resultant product thereof.
In many applications it is desireable to heat laminate a plastic-like protector film to an unprotected underlying sheet for protecting the text material, etc., on the underlying sheet. The sheet and film is subsequently cut into individual pages, covers, etc. However, problems arise in one sided laminations.
Paper is a hygroscopic substance, it picks up excess moisture from the surrounding atmosphere quickly. It's moisture content depends upon the relative humidity and the temperature of the air it is in contact with. An important consequence of the desorption and absorption of water by paper is the change in dimension that usually accompanies changes in moisture content. Dimensional changes in paper originate in the swelling and contraction of the individual paper fibers. It is common for paper to show dimensional changes, up to 1% in the cross direction and corresponding changes in the machine direction could range up to 5%.
Paper used for cover stock for books and magazine covers are typically laminated, and one process of lamination is thermal. The thermal laminating film used usually consists of one layer of polyethylene or copolymer of polyethylene and oriented polypropylene or polyester.
One sided thermal lamination of paper using polypropylene or polyester has some inherent drawbacks. For stock with only one side laminated, moisture is now only absorbed by the paper on one side because of the protection of the laminate film. This increases the dimension of the paper and causes the laminated paper to curl towards the film side or laminated side.
A "perfect" bound book is one in which a layer of adhesive holds the pages and cover together. The problem on typical perfect bound books, depending on the relative humidity of the atmosphere and type of cover stock, can be a slight curl on the cover, i.e., the cover rolls like a scroll.
The protector film can be applied to both sides of the underlying or unprotected product in order to protect text on both sides of the product and to prevent paper moisture growth on any one side compared with the lack of moisture growth of the protector film on the other side. In other words, by laminating on both sides, the various forces are balanced or offset and the resulting web (i.e., product and film) is substantially planar or flat.
However, in most applications it is necessary to protect one side of a product and only one protector film need be laminated to the product. A successful one side lamination is thus desireable.